Stents are generally designed as tubular support structures that can be used in a variety of medical procedures to treat blockages, occlusions, narrowing ailments and other problems that restrict flow through body vessels. An expandable stent is radially compressed to a low-profile configuration for passage through a body vessel, and then, once in position at a treatment site, the stent may be radially expanded to a larger-diameter deployment configuration to contact and support the inner wall of the vessel. Such stents are generally classified as either balloon-expandable or self-expanding and include a framework of interconnected struts. Balloon-expandable stents expand in response to the inflation of a balloon, while self-expanding stents expand spontaneously when released from constraint, such as from a delivery device.
Balloon-expandable stents may provide the benefits of high radial stiffness and strength, minimal recoil, and controlled behavior during expansion. Prior to insertion into a body vessel, a balloon-expandable stent is compressed, such as by crimping, over an uninflated balloon catheter in order to secure the stent to the balloon in a low-profile configuration. For example, compressing or crimping may be done with a handheld tool or by way of a manual or automatic crimping device.
For some applications, it may be advantageous to provide the balloon-expandable stent with a covering or a coating prior to crimping onto the balloon catheter. Such a coating may take the form of a thin biocompatible polymer layer that overlies, underlies and/or encapsulates struts of the stent.
Generally, an uncoated stent is expanded to some fraction of its maximum expanded diameter for application of the covering to minimize the elastic expansion of the coating material that occurs during deployment and expansion of the stent. Covering of the stent can occur by various methods, such as dipping the expanded, uncoated stent into a polymer solution or attaching a thin film, sheet, or tubular structure of material to the expanded uncoated stent. Other processes, such as spray coating or electrospinning, can also be utilized depending on the covering material and the desired morphology of the final covering. After application of the coating, the covered stent may be crimped onto a balloon catheter in preparation for delivery into a body vessel.
Problems may arise during crimping of the covered stent, however, particularly if the stent was expanded to its maximum design diameter prior to the covering process. Crimping a balloon expandable stent after being deformed to an expanded state may result in instabilities during the crimping process that can lead to misalignments or bending of the struts of the crimped stent.